Thin film etching processes fall into two broad categories. One category is conventional liquid phase chemical etching or "wet etching". The other is gas phase plasma-assisted etching or "dry etching".
There are two primary types of dry etch mechanisms: (a) a physical mechanism and (b) a chemical reaction mechanism. In the physical etch mechanism, ions are extracted from a glow discharge and accelerated towards an etch structure whose surface is eroded by momentum transfer upon being hit by the ions. The etch structure typically includes a layer to be etched ("etch layer"), an overlying patterned layer that serves as an etch mask, and an underlying supporting substructure. In the chemical reaction etch mechanism, a glow discharge is employed to generate chemically active ions which diffuse to the etch structure where they react with the surface of the etch structure to produce volatile products.
An important factor related to the type of mechanism used in the etch process is selectivity. Selectivity is a measure of etch rates of different materials. Dry etch processes in which different materials are etched at approximately the same rate are referred to as nonselective. These processes typically use physical etch mechanisms. Dry etch processes in which different materials are etched at substantially different rates are referred to as selective. Some selective etch processes use chemical reaction mechanisms in which chemically reactive ions preferentially react with one material over another. In other selective etch processes, etched material is preferentially redeposited on one material over another.
In some dry etch processes, both types of etch mechanisms are present. In these processes, chemically active ions are extracted from a glow discharge and accelerated toward the etch structure. As a result, the surface of the etch structure is etched by momentum transfer and by chemical reaction.
Reactive ion etching (RIE) is an example of a dry etch process in which both types of etch mechanisms are present. Chemically active ions (reactive ions) are accelerated towards an etch structure which is etched by momentum transfer upon being hit by the reactive ions and by chemical reaction with the reactive ions.
A conventional RIE reactor is schematically shown in FIG. 1. The RIE reactor in FIG. 1 includes a reaction chamber 10 and an electrode 12 capacitively coupled to a high frequency power generator. An etch structure 14 is placed on electrode 12. In operation, a suitable feed gas is introduced into reaction chamber 10, and a glow discharge, shown as region 16, is formed. Since electrons are more mobile than ions, electrode 12 acquires a negative self-bias voltage. Positively charged ions are attracted to electrode 12 and etch structure 14, and reactive ion etching occurs.
A cross-sectional view of a typical etch structure in which a mask 20 overlies an etch layer 22 is shown in FIG. 2a for an RIE process. Mask 20 defines apertures 24 through which etch layer 22 is etched. FIG. 2b is an expanded cross-sectional view illustrating the reactive ion etch of a single aperture 24 in FIG. 2a. R.sup.+ represents reactive ions in FIG. 2b. As reactive ions R.sup.+ travel through mask aperture 24, they collide with the aperture side walls and with other gas molecules. The collisions result in physical and reactive etching as well as recombination with free electrons. As shown, item 26 identifies a region of physical etching, item 28 identifies a region of reactive etching, and item 30 identifies a region of ion-electron recombination.
A deficiency in the reactive ion concentration occurs near the surface of etch layer 22 in apertures with high aspect (depth/width) ratios. Since etch rates are dependent upon reactive ion concentration, the deficiency results in a relatively low etch rate of etch layer 22. This low etch rate, in combination with the relatively high etch rate for mask 20 due to the substantial reactive ion concentration near the aperture opening, results in a loss of selectivity in the etch between etch layer 22 and mask 20 in apertures with high aspect ratios.
Recently there is a trend towards use of low pressure-high density plasmas in dry etch processes. As the name suggests, low pressure-high density plasmas are characterized by high densities of charged and excited species at low pressures. This trend is fueled as minimum feature sizes are reduced to submicrometer dimensions, and aspect ratios increase. Horiike, "Issues and future trends for advanced dry etching," ESC Conference, May 1993 (19 pages), discusses present issues and future trends of dry etching processes employing inductively coupled plasma (ICP), electron cyclotron resonance (ECR), and helicon wave technologies.
In ECR technology, microwave energy is coupled to the natural resonant frequency of electron gas in the presence of a static magnetic field. A conventional ECR waveguide apparatus is schematically shown in FIG. 3. The apparatus includes a waveguide 40 which directs microwave energy 42 into a reaction chamber 50. Process gases are fed into reaction chamber 50. Reaction chamber 50 is surrounded by one or more coils 46 which produce an axial magnetic field. An etch structure 48 is located within reaction chamber 50. Intense electron acceleration is experienced in an ECR layer 52 which sustains the plasma.
In helicon wave technology, a plasma is magnetized longitudinally, and coupling is achieved by a radio frequency (RF) transverse electromagnetic helicon wave. A conventional helicon wave plasma apparatus is schematically shown in FIG. 4. An antenna 60 is used to couple power into a reaction chamber 62. Reaction chamber 62 is surrounded by one or more coils 64 which produce an axial magnetic field. Electrons which resonate with the phase velocity of the helicon wave are accelerated and sustain the plasma. An etch structure 66 is located within reaction chamber 62.
In ICP technology, an inductive element is used to couple energy from an RF power source to ionize gas. A conventional ICP apparatus using a spiral coupler is schematically shown in FIG. 5. The apparatus includes an inductive element 70 to which an RF power source is connected. Inductive element 70 is separated from a reaction chamber 72 by a quartz vacuum window 74. An etch structure 76 is located within reaction chamber 72. As RF current flows through inductive element 70, a time varying RF magnetic flux induces a solenoidal RF electric field within reaction chamber 72. This inductive electric field accelerates free electrons and sustains the plasma. The ICP apparatus shown in FIG. 5 is also referred to as a transformer coupled plasma (TCP) apparatus.
Bassiere et al, PCT Patent Publication WO 94/28569, discloses a method of manufacturing microtips display devices using heavy ion lithography. The method uses a mask that typically consists of polycarbonate for etching a metal gate layer. Bassiere et al cites RIE as an example of an etch process which can be used to etch the gate layer metal. However, use of RIE to etch high aspect ratio apertures inevitably results in substantial degradation of the mask layer due to the dominant nonselective physical etch mechanism. It is desirable to have a process for selectively etching an etch layer, and in particular a metal gate layer, using a polycarbonate mask without significantly eroding the polycarbonate mask.